1. Field of the Invention
The present invention relates generally to a signaling method for use of Multiple Input and Multiple Out (MIMO) in Uplink (UL) of Long Term Evolution (LTE) system and, more particularly, to a method for determining a precoding matrix without a separate control signal.
2. Description of the Related Art
Mobile communication systems have evolved into high-speed, high-quality wireless packet data communication systems that provide data services and multimedia services that far exceed early voice-oriented services. Recently, various mobile communication standards have been developed to support services of the high-speed, high-quality wireless packet data communication systems. These standards include, for example, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), both defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined by the Institute of Electrical and Electronics Engineers (IEEE).
The recent mobile communication systems use specific technologies, such as an Adaptive Modulation and Coding (AMC) method and a Channel-Sensitive Scheduling (CSS) method, to improve the transmission efficiency. Through the use of the AMC method, a transmitter can adjust an amount of transmission data according to a channel state. Specifically, when the channel state is bad, the transmitter reduces the amount of transmission data to adjust a reception error probability to a desired level. When the channel state is good, the transmitter increases the amount of transmission data to adjust the reception error probability to the desired level, thereby efficiently transmitting a large volume of information. Through the use of a CSS-based resource management method, the transmitter selectively services a user having a channel state that is better than those of other users. This selective servicing provides an increase in the system capacity when compared to a method of allocating a channel to one user and servicing the user with the allocated channel. Such a capacity increase is referred to as ‘multi-user diversity gain’. Thus, the AMC method and the CSS method each apply an appropriate modulation and coding scheme at the most-efficient time, which is determined based on partial channel state information that is fed back from a receiver.
Research has been conducted in order to replace Code Division Multiple Access (CDMA), the multiple access scheme used in the 2nd and 3rd generation mobile communication systems, with Orthogonal Frequency Division Multiple Access (OFDMA) in the next generation system. The standardization organizations, such as 3GPP, 3GPP2, and IEEE, have begun standardizations of evolved systems employing OFDMA. The OFDMA scheme results in a capacity increase when compared to the CDMA scheme. One reason for the capacity increase in the OFDMA scheme is that the OFDMA scheme can perform scheduling in the frequency domain (frequency domain scheduling). While the transceiver acquires capacity gain according to a time-varying channel characteristic using the CSS method, the transceiver can obtain a higher capacity gain through the use of a frequency-varying channel characteristic.
In LTE, Orthogonal Frequency Division Multiplexing (OFDM) has been adopted for Downlink (DL) transmissions and Single Carrier Frequency Division Multiple Access (SC-FDMA) has been adopted for Uplink (UL) transmissions. Both transmission schemes are characterized by scheduling on frequency axis.
AMC and CSS are techniques that are capable of improving transmission efficiency when the transmitter has enough information on the transmit channel. In the LTE DL, the base station cannot estimate the DL channel state using the UL receive channel in a Frequency Division Duplex (FDD) mode such that the UE reports the information on the DL channel. However, the DL channel report sent from the UE to the base station can be omitted in a Time Division Duplex (TDD) mode in which the DL transmit channel state is estimated through the UL receive channel. Meanwhile, in the LTE UL, the UE transmits a Sounding Reference Signal (SRS) such that the base station estimates the UL channel using the received SRS.
In the LTE DL, the multiple antenna transmission technique, i.e., MIMO, is supported. The evolved Node B (eNB) of the LTE system can be implemented with one, two or four transmit antennas, and thus, can achieve beamforming gain and spatial multiplex gain by adopting precoding with multiple transmit antennas.
Recently, UL MIMO for LTE has been discussed. In DL MIMO, the eNB, as the transmitter, determines the transmission properties, such as, for example, modulation and coding, MIMO, and precoding schemes. The eNB can configure and transmit a Physical Downlink Shared CHannel (PDSCH) and inform the UE of the transmission property applied to the PDSCH. In UL MIMO, the eNB, as the receiver, determines the transmission properties, such as, for example, modulation and coding, MIMO, and precoding schemes, according to the channel characteristics of each UE. The eNB notifies the UE of the transmission properties through a Physical Downlink Control CHannel (PDCCH). The UE configures and transmits a Physical Uplink Shared CHannel (PUSCH) by reflecting the transmission properties transmitted by the eNB. Specifically, the eNB always makes a decision on the AMC, CSS, and MIMO precoding, and the UE receives the PDSCH and transmits the PUSCH according to the decision made by the eNB.
If the eNB knows the exact channel state, it is possible to determine the amount of data that is most appropriate for the channel state using AMC. However, there is a difference between the channel state known to the eNB and the actual channel state in the real environment due to estimation and feedback errors. Accordingly, it is impossible to avoid errors in actual transmission/reception, even when AMC is applied.
In order to retransmit a signal that failed in its initial transmission, Hybrid Automatic ReQuest (HARQ) is adopted. In HARQ, the receiver sends the transmitter a negative acknowledgement (NACK) indicating a decoding failure on the received data and an acknowledgement (ACK) indicating successful decoding on the received data, such that the transmitter can retransmit the lost data.
In a system using HARQ, the receiver combines the retransmitted signal and the previously received signal to improve the reception performance. The data signal that was previously received and failed in decoding is saved in a memory in consideration of retransmission. The HARQ process is configured such that the transmitter can transmit additional data during the time the ACK or NACK is transmitted by the receiver and the receiver can determine one of the previously received signals to be combined with a retransmission signal based on the HARQ Process IDentifier (HARQ PID). The HARQ can be categorized into one of synchronous HARQ and asynchronous HARQ, depending on whether the HARQ PID is notified by a control signal. In the synchronous HARQ, the HARQ PID is provided in the functional relationship of the subframe sequence number carrying the PDCCH rather than through a control signal. The subframe is a unit of resource allocation on a time axis. In the asynchronous HARQ, the HARQ PID is provided by means of the control signal. The LTE system employs the asynchronous HARQ for DL and the synchronous HARQ for UL.
FIG. 1 is a diagram illustrating a conventional UL synchronous HARQ process.
Referring to FIG. 1, the eNB sends an UL grant in PDCCH in an nth subframe, as shown block 101. The HARQ PID is determined by the subframe sequence n. For example, if the HARQ PID corresponding to the subframe sequence number n is 0, the HARQ PID corresponding to the subframe sequence number n+1 becomes 1. The PDCCH carrying the UL grant in the nth subframe includes a New Data Indicator (NDI). If the NDI is toggled from its previous value, the UL grant is an assignment of PUSCH for a new data transmission. If the NDI is maintained, the UL grant is an assignment of PUSCH for retransmission of previously transmitted data. Assuming that the UL grant of PDCCH 101 is transmitted with the toggled NDI, the UE performs initial transmission of PUSCH carrying new data in an (n+4)th subframe, as shown in block 103. The UE can be aware of whether the PUSCH data transmitted in the (n+4)th subframe has been successfully decoded through a Physical HARQ Indicator CHannel (PHICH) transmitted by the eNB in an (n+8)th subframe, as shown in block 105. If the PHICH carries a NACK, the UE performs PUSCH retransmission in an (n+12)th subframe, as shown in block 107.
As described above, in the synchronous HARQ, the initial transmission and retransmission of a Transport Block (TB) are performed in association with a sequence number of the subframe. Since the eNB and UE know the TB that was initially transmitted in the (n+4)th subframe is retransmitted in (n+12)th subframe, it is possible to perform the HARQ process without the use of a separate HARQ PID. However, since the transmission interval of the same TB is 8 subframes, the number of HARQ processes that can be simultaneously active is limited to 8.
In the UL synchronous HARQ process of FIG. 1, the retransmission is triggered by the PHICH, which indicates only the HARQ ACK or NACK. If it is necessary for the eNB to change the PUSCH transmission property, such as the transmission resource and modulation and coding scheme, for retransmission, it can be allowed to transmit in PDCCH indicating this change. This HARQ scheme allowing for the change of the transmission property is referred to as adaptive synchronous HARQ.
FIG. 2 is a diagram illustrating a conventional UL adaptive synchronous HARQ process.
Referring to FIG. 2, the eNB notifies the UE of a decoding failure of the PUSCH 103 in the (n+4)th subframe by transmitting a NACK in PHICH in the (n+8)th subframe, as shown in block 105. At this time, PDCCH is simultaneously transmitted with PHICH 105 in order to change the transmission property, as shown in block 106. Since PDCCH decoding is attempted in every subframe, the UE can receive the PDCCH 106 for the transmission property change. The UE performs PUSCH retransmission in the (n+12)th subframe based on the transmission property indicated by the PDCCH, in block 108.
In the adaptive synchronous HARQ, even when the amount of DL control information for retransmission increases to an amount that causes overhead, the eNB can transmit PHICH with PDCCH for changing the transmission property or without PDCCH for maintaining the transmission property to minimize the amount of DL control information for the HARQ operation.
FIG. 3 is a flowchart illustrating operations of the eNB for the conventional UL adaptive synchronization HARQ procedure.
Referring to FIG. 3, the eNB performs UL scheduling to allocate a resource for PUSCH transmission to the UE with the UL grant, in step 131. The eNB transmits PDCCH to grant an initial PUSCH transmission to the scheduled UE, in step 133. The eNB receives and decodes PUSCH in the fourth subframe after the subframe where the PDCCH is transmitted, in step 135. The eNB determines whether the PUSCH decoding is successful, in step 137. If the PUSCH is decoded successfully, the eNB sends the UE an ACK, in step 139, and methodology returns to step 131 for new scheduling. If the PUSCH decoding fails in step 137, the eNB sends the UE a NACK, in step 141. According to the adaptive synchronous HARQ operation, the eNB determines whether it is necessary to change the transmission property as compared to that of the initial transmission, in step 143. If it is not necessary to change the transmission property, the methodology returns to step 135 to receive and decode the retransmitted PUSCH. If it is necessary to change the transmission property, the eNB transmits PDCCH to grant PUSCH retransmission having the new transmission property to the UE, in step 145. After transmitting the NACK to request for retransmission, the methodology returns to step 135 to receive and decode the retransmitted PUSCH.
FIG. 4 is a flowchart illustrating operations of the UE for the conventional UL adaptive HARQ procedure.
Referring to FIG. 4, the UE receives and decodes the PDCCH for UL grant, in step 151, and determines whether the PDCCH is decoded successfully, in step 153. If the PDCCH for UL grant is decoded successfully, the UE determines whether the NDI is toggled, in step 155. If NDI is toggled, it indicates that the UL grant is for an initial transmission of a new TB. Thus, the UE transmits a PUSCH carrying the new TB, in step 157. If NDI is not toggled, this indicates that a previous TB having a same HARQ PID was not decoded successfully, and the UE retransmits the PUSCH carrying the previous TB with a transmission property according to an indication of the PDCCH, in step 159.
If the PDCCH for UL grant is not decoded successfully at step 153, the UE receives and decodes a PHICH, in step 161. Upon receipt of the PHICH, the UE determines whether the PHICH carries an ACK, in step 163. If the PHICH carries the ACK, the UE stops transmitting the PUSCH, in step 165. If the PHICH carries NACK, the UE transmits the PUSCH carrying the previous TB with the transmission property indicated by a most recently received PDCCH, in step 167. However, the Redundancy Version (RV) of the PUSCH, which is retransmitted in PHICH, increases automatically without separate instruction.
There are two main schemes for HARQ retransmission: Chase Combining (CB) and Incremental Redundancy (IR). CB is a method that combines an initial transmission and its subsequent retransmission at the symbol level in the receiver. IR is a method for combining an initial transmission and its retransmission having different RVs in the decoding process of the receiver. In spite of its high complexity as compared to CB, the IR is widely used for HARQ retransmission due to the additional decoding gain. Since the PDCCH for changing the RV in the synchronous HARQ is not transmitted, the RV is determined implicitly. In the LTE system, a total of 4 RVs are defined (RV=0, 1, 2, 3). In case of the synchronous HARQ, the RV is applied in order of {0, 1, 2, 3}, according to the transmission order.
The Downlink Control Information (DCI) for UL grant for the PUSCH transmission includes the following Information Elements (IE):                A flag for differentiating between DCI format 0 and DCI format 1A: Because DCI format 0 for UL grant and DCI format 1A for compact DL assignment are always forced to be the same size in LTE, there is a need to differentiate between format 0 and format 1A.        Frequency hopping flag: This flag is an IE used to notify of the use of frequency hopping for frequency diversity in PUSCH transmission.        Resource assignment information: This IE is defined for indicating the resource assigned for PUSCH transmission.        Modulation and Coding scheme: This is an IE that indicates the modulation and coding scheme for use in PUSCH transmission. Some codepoints of this IE are defined to indicate the RV for retransmission.        NDI: This is an IE indicating whether the corresponding grant is an initial transmission of a new TB or a retransmission. If its value is toggled, it indicates a grant for a new TB transmission and, otherwise, it indicates a grant for retransmission.        Transmit Power Control: This is an IE indicating the transmit power for use in PUSCH transmission.        RS parameter Cyclic Shift Index (CSI): The RS for PUSCH demodulation is defined with a Zadoff-Chu (ZC) sequence. The ZC sequence has a characteristic in which the new ZA sequence is acquired by changing the cyclic shift. The IE indicating the cyclic shift of the RS for PUSCH demodulation is defined in the UL grant for multiuser MIMO. By assigning the RSs having different cyclic shift indices, the eNB can discriminate between the signals of different users based on the orthogonality of the RS.        Channel Quality Indicator (CQI) request: This is an IE for requesting non-periodic CQI feedback on the PUSCH. This IE is 1 bit, and is set to 1 for transmission of non-periodic CQI, a Precoding Matrix Indicator (PMI), and a Rank Indicator (RI) along with data; and is set to 0 for data transmission only on the PUSCH.        
Unlike the case where both of the two TBs are successfully decoded or not decoded, the eNB requesting UL MIMO transmission can define the precoding operation of the UE without transmission of PDCCH. If one of the two TBs is successfully decoded, it is necessary to transmit the PDCCH indicating the precoding scheme of the UE. This characteristic degrades the significant advantage of the synchronous HARQ. The synchronous HARQ can trigger retransmission only with PHICH, and without transmission of PDCCH. Unlike the PHICH carrying only the ACK/NACK information, PDCCH is designed to carry various control information such that the eNB consumes a relatively large amount of frequency resources and transmission power for PDCCH transmission. Specifically, one of the advantages of the synchronous HARQ is to minimize the frequency resource and transmission power consumption. Accordingly, the PDCCH transmission for retransmission grant causes an increase of resource consumption for the control signal.